Human respiratory syncytial virus (RSV) infects nearly everyone worldwide early in life and is responsible for considerable mortality and morbidity. In the United States alone, RSV is responsible for 75,000-125,000 hospitalizations yearly, and worldwide conservative estimates conclude that RSV is responsible for 64 million pediatric infections and 160,000 pediatric deaths. Another unusual feature of RSV is that severe infection in infancy can be followed by years of airway dysfunction, including a predisposition to airway reactivity. RSV infection exacerbates asthma and may be involved in initiating asthma.
RSV is a member of the Paramyxoviridae family and, as such, is an enveloped virus that replicates in the cytoplasm and matures by budding through the host cell plasma membrane. The genome of RSV is a single, negative-sense strand of RNA of 15.2 kilobases that is transcribed by the viral polymerase into 10 mRNAs by a sequential stop-start mechanism that initiates at a single viral promoter at the 3′ end of the genome. Each mRNA encodes a single major protein, with the exception of the M2 mRNA, which has two overlapping open reading frames that encode two separate proteins. The 11 RSV proteins are: the RNA-binding nucleocapsid protein (N), the phosphoprotein (P), the large polymerase protein (L), the attachment glycoprotein (G), the fusion protein (F), the small hydrophobic (SH) surface glycoprotein, the internal matrix protein (M), the two nonstructural proteins NS1 and NS2, and the M2-1 and M2-2 proteins encoded by the M2 mRNA. The RSV gene order is: 3′-NS1-NS2-N-P-M-SH-G-F-M2-L. Each gene is flanked by short transcription signals called the gene-start (GS) signal, present on the upstream end of the gene and involved in initiating transcription of the respective gene, and the gene-end (GE) signal, present at the downstream end of the gene and involved in directing synthesis of a polyA tail followed by release of the mRNA.
Vaccines and new antiviral drugs are in pre-clinical and clinical development; however, no vaccines for RSV are commercially available yet. The goal of the present study was to design and generate new vaccine candidates for RSV by using the recently described synthetic attenuated virus engineering (SAVE) technique. (Coleman, et al., Science 320:1784-1787 (2008)). This technique is used to recode a genome in which the wild type (wt) amino acid sequence is unmodified, but synonymous codons are rearranged to create a suboptimal arrangement of pairs of codons that deviates from the natural frequency of occurrence of certain codon pairs. For pathogens, the attenuation resulting from this rearrangement of codons can be ‘titrated’ by adjusting the extent of codon-pair deoptimization (CPD). Recombinant pathogens that were attenuated by this approach encode proteins with wt aa sequences. Thus, these pathogens are likely to induce a cellular and humoral immunity against the same epitopes as the wt pathogen.